This invention relates to a power switching circuit having a power semiconductor switch, which circuit has particular, but not exclusive, application in the automotive industry.
The power semiconductor switch of such a power switching circuit comprises a plurality of parallel-connected active cells, source cells where the power semiconductor switch is a power MOSFET or an insulated gate bipolar transistor (IGBT).
As disclosed in, for example, EP-A-0 139 998 it is known to separate the active cells into a main current carrying section containing the majority of the active cells and a sense or emulation current carrying section containing only a few of the active cells by, in the case of a power MOSFET or IGBT, providing separate connections to the sources of the active cells of the main current carrying section and of the sense current carrying section. As disclosed in EP-A-0 139 998, the current flowing through the main current carrying section should be related to that flowing through the sense current carrying section by a constant K given by the ratio of the active areas of the main current carrying and sense current carrying sections, that is the ratio between the number of active cells in these two sections when all the active cells are identical. Therefore sensing the current flowing through the current carrying section should enable a determination to be made as to the current flowing through the main current carrying section of the device. This enables, for example, detection of faults in the power semiconductor switch itself or the detection of, for example, a short or open circuit when the load which the power switch is arranged to switch is defective.
However, under certain circumstances, for example change in the input impedance of the circuit used to sense the current flowing through the sense current carrying section or in the input impedance of the load connected to the semiconductor power switch, the current flowing through the sense current carrying section may not be proportional to that flowing through the main current carrying section. U.S. Pat. No. 5,081,379 addresses this problem by providing a voltage impression device which impresses the voltage of the source electrode (in the case of a power MOSFET or IGBT) of the main current carrying section on the source electrode of the sense current carrying section.
As disclosed in U.S. Pat. No. 5,081,379, the voltage impression device comprises a differential amplifier having its non-inverting input connected to the source electrode of the main current carrying section and its inverting input connected to the source electrode of the sense current carrying section. The output of the differential amplifier is connected to the control electrode of control transistor (a p-channel IGFET (insulated gate field effect transistor) in the example given where the power switch is an n-channel power MOSFET)) having its source electrode connected to the source electrode of the sense current carrying section of the power semiconductor switch.
Even in the power switching circuit disclosed in U.S. Pat. No. 5,081,379 the current flowing through the main current carrying section is not exactly related to measured current by the factor K, rather the current through the main current carrying section is related to the measured current by:
K.(1xe2x88x92Voff/Vbl)
where Voff is the input offset voltage of the differential amplifier and Vbl is the voltage at the load with respect to the positive supply voltage where the power semiconductor switch is a high side switch for connecting a load to a positive voltage supply line (such as the positive terminal of the battery where the power switching circuit is used in an automotive application).
At high current levels Vbl is significantly bigger than Voff and so the error, Voff/Vbl, in the measure of the current through the load obtained by sensing the current through the sense current carrying section is small. However, at low current levels, the amplifier input offset voltage Voff presents a significant part of the load voltage and gives rise to a significant error. Previously, this problem has been addressed by increasing the load voltage by reducing the voltage applied to the control electrode of the power semiconductor switch relative to the load voltage Vbl, thereby increasing the on-resistance of the power switch. The problem with this approach is that reducing the drive voltage to the control electrode in order to increase the on-resistance of the power semiconductor switch takes the power switch out of its linear operation region so that the relationship between the sensed current and that flowing through the main current carrying section becomes dependent on the difference between the control electrode and offset voltage squared resulting in the current through the main current carrying sections being related to the measured current by:
K.(Vgsxe2x88x92Vtxe2x88x92Voff)2/(Vgsxe2x88x92Vt)2
where Vt is the threshold voltage of the power switch and Vgs is the voltage between the control and source electrodes of the main and sense current carrying sections. As can be seen from a comparison of equations 1 and 2, this approach results in the offset voltage having an even more significant effect giving rise to an even greater error as the sense error has terms proportional to Voff and Voff squared.
Such problems may be reduced, but not eliminated, by using a high precision operational amplifier with an extremely low offset voltage. This would, however, significantly increase the cost of the power switching circuit.
It is an aim of the present invention to provide a power switching circuit which enables errors in the measure of the current through the main current carrying section arising from the voltage offset of the amplifier to be avoided or at least reduced without the necessity for using an extremely high precision operational amplifier.
In one aspect, the present invention provides a power switching circuit comprising a power semiconductor switch, which may be intended to operate as a high-side switch, having a main current carrying section and a sense current carrying section for carrying a smaller current than the main current carrying section, the first and second current carrying sections sharing a common first main electrode, the main current carrying section having a main current carrying electrode for connection to a load and the sense current carrying section having a sense current carrying electrode for enabling the current through the sense current carrying section to be measured, wherein at least the main current carrying section is formed of a plurality of subsidiary current carrying sections, means are provided for sensing the voltage at the main current carrying electrode and for switching off or reducing the current through at least one of the plurality of subsidiary current carrying sections of the main current carrying section when the voltage at the main current carrying electrode drops below a predetermined value so as thereby to increase the resistance through the power semiconductor switch device while maintaining the power semiconductor switch in its linear operation region.
According to one aspect of the present invention, there is provided a power switching circuit as set out in claim 1.
This enables accurate measurement of the current flow through the power semiconductor switch at low current levels by increasing the on-resistance in a manner which allows the power semiconductor switch device to remain in its linear operation region.
Various preferred features in accordance with the invention are set out in claims 1 to 15.